1. Field of the Invention
This invention relates to vacuum cleaners and in particular to means for indicating a condition of operation of a vacuum cleaner.
2. Description of the Prior Art
In the conventional vacuum cleaner, air is sucked by a suitable suction means to pick up dirt from a surface being cleaned. The air with the entrained dirt is flowed through a dirt-collecting means conventionally in the form of a porous bag which removes the entrained dirt and passes the cleaned air back to the atmosphere. The air suction means is conventionally powered by an electric drive motor. The power consumption of the motor varies as the bag pressure on the suction means. Such bag pressure conventionally increases as the dirt-collecting means becomes more and more filled with dirt until, in a substantially clogged condition, the bag pressure may be sufficient to cause the power consumption of the motor to overheat the motor and cause possible damage to the vacuum cleaner.
To avoid such damage, a number of devices have been developed which automatically terminate operation of the drive means when the dirt-collecting means becomes substantially clogged. One such structure is shown in U.S. Pat. No. 2,715,452 of Moss A. Kent. Another such structure is shown in U.S. Pat. No. 3,510,904 of Robert C. Lagerstrom. In each of these patents, the temperature of the air as heated by the electric drive means is sensed by a suitable thermostat to open the circuit to the drive means motor.
In another form of vacuum cleaner device, the prevention of overheating of the motor is effected by means of a pressure switch. Such pressure switch controls, however, have the disadvantage of nuisance operation as a result of use of bayonet cleaning tools and the like which tend to restrict the air flow in the normal operation of the vacuum cleaner notwithstanding a substantially unclogged condition of the dirt-collecting means.